Metals having improved machinability and method

ABSTRACT

THE IMPROVEMENT IN MACHINABILITY OF STEELS AND OTHER METALS BY THE INCLUSION OF FOREIGN BODIES HAVING A PREDETERMINED SIZE AND DISTRIBUTION WITHIN THE METAL.

Dec. 5, 1972 E. s. NACHTMAN 3,705,020

METALS HAVING IMPROVED MACHINABILITY AND METHOD Filed Feb. 2, 1971 g 20A E'G. 1

Quench and i3 150-- 5120- iv! I g 80* 007 00002 M115 R 3 so A A Alormalgai I E 3 20 S i l l PARTICLE SIZE. MICRONS Machmablllay Trend Lines kepreseniirzxigrfikrenf Purim/e Sizes of Addllfl ts a? 778 Volume Percenf inSiceZ chr'nedaf Two Di/Ysreni Cultmy Speeds FI G. 2 1.10 0%; 0 2 Volume MrLS' oo 75!]0111 Alormafized Sieel A- A 200 fpm Normalized Szeel 75f 0 bed and mp 'g l Steel 40, Pariiclc Diqmefer (assumed Spherical) 70" ZZOO pflimum Gow- l l 1 l e l 7 0 1000 2000 4000 0000 1 000 20,000

CR1 CULA ZED INTEQ PR5 T/CLE SPACLALC'II [I g g P555 P ll A lClQ N5 Effecfof lnigrpnrficle Spacing 0f 40-Micg-0n MnS Padre/e5 as free Machzrzmg Add: i: me on Machmabzlzzg 1 v v E/v TOR Rafmgs of /045 steel United States Patent 3,705,020 METALS HAVING IMPROVED MACHINABILITY AND METHOD Elliot S. Naclitman, Evanston, Ill., assigns! to LaSalle Steel Company, Hammond, Ind. Filed Feb. 2, 1971, Ser. No. 111,916 Int. Cl. B32f 7/00 US. Cl. 29-1823 16 Claims ABSTRACT OF THE DISCLOSURE The improvement in machinability of steels and other metals by the inclusion of foreign bodies having a predetermined size and distribution within the metal.

This invention relates to the improvement of the machinability of metals and it relates more particularly to steels having improved machinability characteristics.

To the present, steels having improved machinability have been produced by the addition of lead to steel in the production of leaded steels. Other free machining additives which have been added to steels to improve machinability include tellurium, bismuth, sulphur and selenium. In my issued US. Patent No. 2,789,069, description is made of the unexpected improvements which have been experienced in the machinability of steels by the intentional inclusion of copper, and in my issued US. Patent No. 3,094,440, description is made of the additions of zinc as a free machining additive to steel. Such additives are believed to function in the manner of an internal lubricant or to provide the cutting tool with a coating to reduce Wear and failure whereby an apparent improvement is secured in the machinability characteristics of the metal.

While these free machining additives are effective to improve machinability, they incidentally often undesirably atfect others of the physical and mechanical properties of the steel. Thus it is an object of this invention to produce and to provide a method for producing steels and other metals having improved machinability characteristics without detriment to others of the mechanical and physical properties of the steel or other metal.

This invention is addressed to the discovery of an entirely new and novel concept in the means by which the machinability characteristics of steels might be effected. It departs completely from internal lubrication of the steel or the formation of a protective coating on the tool, as evidenced by the fact that the concept of this invention can be demonstrated by modification of the steel to include additives, such as aluminum oxides and the like materials which normally would be expected to increase the hardness of the steel, increase wear on the cutting tool, interfere with internal lubrication, or in other words reduce the machinability characteristics of the steel.

It has been found that size and distribution of foreign bodies (inclusions) in the steel has an unexpected and material effect on the machinability characteristics of the steel. By proper selection of both size and distribution of the inclusions, steels having noticeable improvement in machinability can be produced substantially independently of the chemistry of the inclusions. By proper selection of both the materials and their size and distribution in the steel, improvements can be achieved in the machinability characteristics with minimum effect on others of the mechanical ad physical properties of the steel. In fact, by proper selection of materials having the desired size and distribution in the steel, others of the mechanical properties of the steel can be improved, such as hardness, strength, ductility, surface finish and the like, in addition to marked improvement in the machinability characteristics of the steel.

The unique character of this invention can perhaps best be established by the fact that improvement in machinability characteristics of steels can be achieved by inclusions of such compounds as manganese sulfide, aluminum oxide and the like materials which, based upon current theories, would normally be expected to increase the hardness and brittleness of the steels and materially reduce their machining characteristics. Thus it was entirely unexpected to find that when aluminum oxide particles are incorporated in the desired size and distribution in the steel, marked improvements can be secured in machinability in addition to other improvements otherwise normally expected to be derived from the use of such materials.

The amount of inclusions is not a significant factor since the volume percent of the inclusions will depend somewhat upon the size of the inclusions and the distribution of the inclusions in the steel. It has been found that the desired effect on machinability of steels can be secured when the size of the inclusions of particles is within the range of 10 to microns and preferably 20 to 60 microns and their distribution, as measured by interparticle spacing (Mean Free Path) is within the range of 1000 to 4000 microns and preferably 1600 to 2400 microns. The range and the optimum will vary slightly from the above, depending somewhat upon the chemistry of the inclusions.

By way of modification, it has also been found that the physical shape of the inclusions has a correlation with the machinability rating that is obtained with a particular inclusion. For example, disc shaped inclusions, obtained as by cross rolling of sheets or plates containing the in clusions, gives relatively uniform machinability ratings in both directions. On the other hand, rod-like inclusions, obtained by unidirectional rolling or elongation of the work piece containing the inclusions, gives directional variation in machinability.

As used herein, the term inclusions refers, in its broadest sense, to foreign bodies within the metal matrix which, by reason of being foreign to the metal, are capable of separate identification from the standpoint of size and distribution, such as inorganic inclusions of the type silicon carbide, cadmium sulfide, aluminum oxide, zirconium oxide, molybdenum disulfide and the like highly insoluble, high melting point inorganic compounds or metallic compounds. Included also as representative are the free machining additives such as lead, bismuth, selenium, tellurium and sulphur which becomes even more effective in their improvement of machinability, when size and distribution of the metal or compounds formed thereof are controlled as described herein. In its broadest sense, the term would also include voids within the metal of the desired size and distribution since such voids are foreign bodies which yield the desired improvements in machinability when present in the desired size and distribution in the particular metal.

There are various techniques by which such inclusions might be incorporated to achieve the distribution of inclusions of desired size in the metal matrix. The technique which becomes immediately apparent is by the process of powdered metallurgy wherein the inclusions in the desired amounts and size can be uniformly mixed with the powdered metal and pressed into compounds which can then be sintered and forged or otherwise worked to improve the density.

More highly specialized techniques are required to be employed to achieve the desired size and distribution of the inclusions in a metal matrix by addition while the metal is in a molten state. This can be accomplished without difficulty merely by mixing, when the inclusions are insoluble or incompatible with the molten metal or enjoy a melting point above the temperature of the metal of the matrix. When the inclusion is soluble in the metal or capable of alloying with the metal or being otherwise modified by the molten metal, it is desirable to withhold cent of a mixture of equal parts of alumina and manganese sulfide. While the effects secured with alumina or with a mixture of alumina and manganese sulfide were not as pronounced as with manganese sulfide alone, the

the addition of the inclusions until the molten metal 5 tests indicated that the particle size which is effective to closely approaches its solidification temperature. As forincrease machinability characteristics of the steel lies geneign bodies, the inclusions can be generated in situ in the eraliy within the same range as that for manganese sulfide. desired size and distribution by separation or by precipi- The effect of spacing of the inclusions on the machintation from the molten metal in response to certain preability characteristics of steel can be demonstrated by the cipitation or insolubilization factors, such as temperature following example using manganese sulfide at the optichange, elemental additions, gaseous additions, pH change mum particle size of 40 microns, as determined in Exand the like. Such foreign bodies, formed in situ by ample l, but in which the amount of manganese sulfide separation from the metal matrix, will of course remain was varied to provide compounds with different spacing uniformly distributed throughout the metal upon between the particles. hardening.

The effect of particle size on the machinability of the EXAMPLE 2 steel is illustrated by the following example, using a 1040 Again powdered metal compounds were prepared with steel as the matrix and manganese sulfide of various ar- 1040 steel in which manganese sulfide, having an averticle sizes admixed with the steel in the amount of one age particle slze of 40 microns, was admlxed in amounts volume per cent. ranging from 0.2 to 3.0 volume percent. The compounds EXAMPLE 1 were forged into round bars and heat treated as in Example 1 and then subjected to the same machinability gft g fi zzfi 5312 1222? z gi zgg gg mz gig tests at cutting speeds of 75 and 200 feet per minute, as volume percent of manganese sulfide particles ranging in in Example 1, to determine their machinability ratings. particle size from 0.001 to 100 microns. For control, g results are Set forth In the curve ldgmlfied as Fompqunds were made of the Same met-a1 without this: The interparticle spacing is identified by their mean inclusions The comPoundS were forged lightly at free path (MFP) as calculated from the formula for F. to round bars wh1ch were then heated by normalizmg g herical articles or by quenching and tempering. p

The bars were tested for machinability at cutting speeds M F 2 l( l: Q of 75 and 200 feet per minute by the standard machin- 3f, i hefemafler to be descnbed which main which a' is the diameter of the particle and i is the chmabrlity ratings for the test bars as represented in the volume fraction. accomqamgng grap? ldentlified as i From the values represented in the curves, it will be i wln mm t e wives t w i seen that rapid improvement in machinability is experiablhty ratings for l normahzeq 1040 steel Increase enced with an interparticle spacing of 800 microns. Imgraquany i f 8 i manganie. Sulfide provement continues until the interparticle spacing ex- }.lavmg i?" e fslze o b T 9" a g mcrease ceeds about 6000 microns with best results being secured ma a out figms manganese 40 with interparticle spacing of 1600-2400 microns, with an sulfide ll'iClUSlOIlS having a particle size of 3 .i'l'llCl'Ol'lS and Optimum in an casfis lying between 2009 and 2200 that an optimum rating of about 120 is reached in the microns range of 30-40 microns followed by a rapid falling oif as EXAMPLE 3 the manganese sulfide particles exceed 60 microns and I I approach 100 microns in size. Between 3 and 30 microns, The effect of composltlon, size and d stribution of manthe machinability rating increases about from a ganese sulfide particles on the mechanical properties and rating of less than to a rating of 120. It will be signifimachinabilitycharacterlstics of mcdiumcarbon steels were cant that the cutting speed has little if any effect on the determmedwrth specimens prepared as in Examples 1 and machinability rating of the tested teels. 2, W1th variations in volume pecent for rnterparticle spac- Similar tests were conducted on normalized and 50 mg of the manganese sulfide, with the machinability ratquenched-and-tempered bars containing one volume permgs being determined by a constant pressure lathe test, cent alumina and other bars containing one volume perwlth the following results:

TABLE I Composition MnS additive Mechanical properties M Mean Roduc- Machin- S Difree Tensile Yield Elongaticn in ability Total, Free, (total), Volume ameter, distance, strength, strength, tion, area. rating, AISI steel percent percent percent percent percent microns microns p.s.i. p.si percent percent percent 0. 4s 1. 04 0. 14 0. 52 2. s as 764 72. 7 36. 7 20 20 104 0. 49 0. s0 0. 34 0. 3 1. a a2 1. 312 so. 1 46. 1 25 a3. 4 125 0. 51 0. 77 0. 4a 0. 22 0. 05 2s 1, s07 02 47. 7 26 30 126 0. 52 0 0.176 0. 54 24 2,047 00. 7 52.0 22 38.5 0. 51 0 0. 062 0. 24 20 5, 542 00. 2 53. 8 2s 50 72 0. 4a 0 105. s 55 24 44 66 0 30 0 102.7 53.8 23 45.3 62 Random iron TABLE 11 Composition MnS additive Mechanical properties Mn Average Mean Reduc- Machin- S diatnfree Tensile Yield Elonga tion in ability C, Total, Free, (total), Volume etcr, distance, strength, strength, on, area, rating, AISI steel percent. percent percent percent percent microns microns p.s.i. p.s.i. percent percent percent;

J Random iron In general, it will be seen from the above data that increase in content of manganese sulfide results in lower reduction in area values in tensile tests with progressively higher machinability ratings. For nearly the same content of manganese sufide, the machinability improves with increase in particle size. Table II further indicates that higher strengths and ductility do not significantly impair improvement in machinability, provided the size and distribution of the particles is Within the range indicated.

Briefly described, the tests for machinability ratings were conducted in each case by taking a /3 inch turn of cut on each bar tested. The cross feed was produced by a horizontal thrust force originating from a constant weight (7 pounds) operating through a pulley system. Cutting speeds of 75 f.p.m. and 200 f.p.m. were used for the bars and for the controls.

The steels were each subjected to four or five tests with a complete test consisting of cutting a group of test samples along with a standard control with cuts on the standard as a control at the beginning and at the end of each test. To compensate for variations which might exist in cutting tool conditions, a new cutting tool was used for each of the separate test groups with the order in which the samples were machined being changed for each successive test so that the reported average would take into consideration differences in cutting edge conditions during the progress of the series of cuts with the same tool.

It appears that these same concepts for improvement in machinability by inclusion of foreign bodies having the desired spacing and distribution can be produced in metals other than steel, such as, for example, in aluminum, titanium, zirconium, molybdenum, nickel, cobalt and the like, or alloys such as high alloy steels, super alloys of nickel and cobalt and the like.

It will be understood that changes may be made in the details of formulation and operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. Metals having improved machinability characteristics by reason of the inclusion of foreign bodies uniformly distributed throughout the metal of a size within the range of 10 to 100 microns and a spacing between the inclusions averaging within the range of 1000 to 4000 microns.

2. Metals having improved machinability characteristics by reason of the inclusion of foreign bodies uniformly distributed throughout the metal of a size within the range of 20 to 60 microns and a spacing between the inclusions averaging within the range of 1600 to 2400 microns.

3. Steels having improved machinability characteristics by reason of the inclusion of foreign bodies uniformly distributed throughout the steel of a size within the range of 10 to 100 microns and a spacing averaging within the range of 1000 to 4000 microns.

4. Steels having improved machinability characteristics by reason of the inclusion of foreign bodies uniformly distributed throughout the steel of a size within the range of 20 to 60 microns and a spacing averaging within the range of 1000 to 4000 microns.

5. Steels having improved machinability characteristics as claimed in claim 3 in which the inclusions are disc shaped to provide corresponding improvements in machinability in two directions.

6. Steels having improved machinability characteristics as claimed in claim 3 in which the inclusions are rod shaped with corresponding improvement in machinability in the direction in which the rod extends in the steel.

7. Steels having improved machinability characteristics as claimed in claim 3 in which the foreign bodies are selected from the group consisting of silicon carbide, cadmium sulfide, aluminum oxide, zinc oxide and molybdenum disulfide.

8. Steels having improved machinability characteristics as claimed in claim 7 in which the steel has a total sulphur content of not more than 0.25 percent.

9. In the method of improving the machinability of steel comprising the step of admixing foreign particles with the steel to provide inclusions uniformly distributed throughout the steel having an average size within the range of 10 to 100 microns in an amount sutiicient to control the volume fraction (f,,) of the foreign bodies in the steel to provide average spacing between the inclusions within the range of 1000 to 4000 microns as determined by the equation wherein d is the average diameter of the foreign particles.

10. The method as claimed in claim 9 which includes the step of working the steel in two mutually perpendicular directions to form the inclusions to disc shape to improve machinability in both directions.

11. The method as defined in claim 9 wherein the foreign particles have an average size within the range of 20 to 60 microns and the volume fraction thereof is controlled to provide an average spacing of 1600 to 2400 microns.

12. The method as claimed in claim 9 which includes the step of working the steel in one direction to elongate the inclusions to rod-like shape to provide directional improvement in machinability.

13. The method as claimed in claim 9 in which the inclusions are selected from the group consisting of silicon carbide, cadmium sulfide, aluminum oxide, zinc oxide, and molybdenum disulfide.

14. The method as claimed in claim 9 wherein the foreign particles are in the form of a powder and are admixed with the steel powder, the resulting mixture is formed into compacts and the compacts are densified to form a composite metal.

15. The method as claimed in claim 14 in which the compound is densified by forging.

16. The method as claimed in claim 15 which includes the step of heat treating the compact after densification.

References Cited UNITED STATES PATENTS 3,094,440 6/1963 Nachtman 1484 3,278,281 11/1966 Ehringer 206 3,440,042 4/ 1969 Kaufman 75206 OTHER REFERENCES Correlation of Machinability With Inclusion Characteristics in Resulphurized Bessemer Steels, Van Vlack, ASM 1952.

CARL D. QUARFORTH, Primary Examiner R. E. SCI-IAFER, Assistant Examiner US. Cl. X.R. 

